1. Field of the Invention
This invention relates to an etching process and composition for forming patterns in a polyimide or similar polymeric insulating layer on a semiconductor device. More particularly, it relates to an improved process for etching such patterns which will allow either a positive or a negative photoresist to be used to define the patterns to be etched, and which does not require the use of hazardous materials or temperatures in the etching process.
2. Description of the Prior Art
The use of polyimide and related polymers as insulating materials for flexible printed circuits and similar applications is now well known. In the fabrication of such printed circuits, it is necessary to both bond a polyimide film to a copper film and to etch openings in the polyimide for contacting the copper film. A wide variety of inorganic and organic bases and amines have been used in the prior art, both as surface treatments for the polyimide film prior to bonding it to the copper, and as etchants for forming openings in the polyimide film after bonding to the copper. Examples of such surface treatment and etching processes and compositions are disclosed in U.S. Pat. Nos. 3,361,589; 3,770,528; 3,791,848; and 3,871,930.
More recently, it has also become known to utilize polyimide and related polymers, such as polyimide-iso-indroquinazolinedione to form insulating layers on semiconductor devices, such as integrated circuits. It is also necessary to form contact openings through such polyimide insulating layers in integrated circuits. In contrast to the printed circuit application, many of the openings in the insulating layers employed in integrated circuits are much smaller, such as 4 microns or less. Such small openings must often be be formed simultaneously with larger openings which will allow bonding pads to contact underlying aluminum metallurgy in the integrated circuits. Because the aluminum metallurgy is itself fairly thin, care must be made not to use an etchant for the polyimide insulating layer which will also attack the underlying aluminum. The need to avoid attack on the aluminum and the need to etch both very small holes and larger holes simultaneously with accurate definition means that many of the etchants employed for etching polyimide in flexible printed circuits are not suitable for use in etching polyimide insulating layers on integrated circuits.
One approach that has been suggested to avoid the problem of attacking aluminum metalization in integrated circuits is to etch openings in polyimide through the use of oxygen plasma, for example, as employed to strip photoresist from integrated circuits. In one such process, the oxygen plasma removes photoresist having a pattern of openings developed in it to define areas of the polyimide where etching is desired at the same time that the polyimide is etched where it is exposed. Such a process is described in U.S. Pat. No. 3,767,490. Another approach taught in the prior art for etching patterns in polyimide insulating layers of integrated circuits is to use an etching solution consisting of hydrazine and a diamine for either partially cured polyimides or fully cured polyimides. Such processes are described in U.S. Pat. Nos. 4,113,550 and 4,218,283. However, hydrazine is a very hazardous material, and most manufacturers of integrated circuits are reluctant to employ it. As a result, there has been a tendency to employ inorganic and organic bases as etchants, even though they are prone to attack aluminum. It has been difficult to etch different sized openings simultaneously with such etchants and the slope of openings etched with such etchants has tended to be non-uniform. Such processes also often employ an elevated temperature, which tends to increase the hazard of an otherwise acceptable etchant.
Thus, there is a need for further development of etching processes and compositions for forming openings in polyimide and related polymer insulating layers on integrated circuits.